The present invention relates to a silicone rubber composition for fixing rolls used in copier machines, printers, fax machines, etc. and to a fluorocarbon resin-covered fixing roll. More specifically the present invention relates to a liquid silicone rubber composition for fixing rolls capable of forming silicone rubber of superior durability and hardness stability under high-temperature conditions and which exhibits practically no decrease in hardness after high-temperature treatment, and to a fluorocarbon resin-covered fixing roll of superior hardness stability and reliability during use.
Fixing rolls produced by forming a low-hardness silicone rubber layer over the peripheral surface of a metal roll shaft and then coating this external layer with a fluorocarbon resin are called fluorocarbon resin-covered fixing rolls. They are widely used in electrophotographic copiers, printers, facsimile machines, etc. (refer to Japanese Publication No. Sho 53(1978)-74436, Japanese Publication No. Sho 57(1982)-89785, Japanese Publication No. Sho 59(1984)-74578, and Japanese Publication No. Sho 59(1984)-52269). Superior heat resistance is required of this type of fluorocarbon resin-covered fixing rolls because they are used for fixing toner at high temperatures in excess of 200xc2x0 C. Because the silicone rubber of the underlayer portion is shielded from contact with the air by the fluorocarbon resin coating, this tends to cause depolymerization, brings about a decrease in hardness, generates non-uniformity in hardness, and other problems. In addition, when the process of providing a coating of fluorocarbon resin on the surface of a roll is based on methods involving application of a fluorocarbon resin latex and baking at a high temperature, the silicone rubber of the underlayer undergoes softening and degradation because during the high temperature baking step of about 220xcx9c350xc2x0 C. When this type of silicone rubber is employed for fluorocarbon resin-covered fixing rolls, during paper-passing durability testing, toner residue adheres to the surface of the roll, or creasing develops in the fluorine material layer on the surface of the roll.
As a result of in-depth investigations directed to eliminating the problems of the above-described prior art, the authors of the present invention found that a liquid silicone rubber composition produced by the addition of a particulate hydroxide or oxide of an alkali metal or a particulate hydroxide or oxide of an alkaline earth metal is superior in heat resistance, and especially heat stability in the absence of air due to a fluorocarbon resin coating. It is an object of the present invention to provide a silicone rubber composition for fixing rolls capable of forming silicone rubber of superior durability and hardness stability that exhibits practically no decrease in hardness after high-temperature treatment in a fluorocarbon resin coating step, and to provide a fixing roll of superior hardness stability and reliability during use.
A liquid silicone rubber composition for fixing rolls comprising
(A) 100 parts by weight of a diorganopolysiloxane which has at least two silicon-bonded alkenyl groups in each molecule and is liquid at room temperature,
(B) 5xcx9c300 parts by weight of an inorganic filler,
(C) an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule in a quantity that affords a value of from 0.3:1 to 5:1 for the ratio of the number of moles of silicon-bonded hydrogen atoms in the present component and silicon-bonded alkenyl groups in component (A),
(D) a platinum catalyst in a quantity that affords a value of from 0.1 part by weight to 500 parts by weight of platinum metal atoms per 1,000,000 parts by weight of component (A), and
(E) 0.01xcx9c10 parts by weight of a particulate hydroxide or oxide of an alkali metal, or a particulate hydroxide or oxide of an alkaline earth metal.
The present invention is a liquid silicone rubber composition for fixing rolls comprising
(A) 100 parts by weight of a diorganopolysiloxane which has at least two silicon-bonded alkenyl groups in each molecule and is liquid at room temperature,
(B) 5xcx9c300 parts by weight of an inorganic filler,
(C) an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule in a quantity that affords a value of from 0.3:1 to 5:1 for the ratio of the number of moles of silicon-bonded hydrogen atoms in the present component and silicon-bonded alkenyl groups in component (A),
(D) a platinum catalyst in a quantity that affords a value of from 0.1 part by weight to 500 parts by weight of platinum metal atoms per 1,000,000 parts by weight of component (A), and
(E) 0.0110 parts by weight of a particulate hydroxide or oxide of an alkali metal, or a particulate hydroxide or oxide of an alkaline earth metal.
The diorganopolysiloxane of component (A), which has at least two silicon-bonded alkenyl groups in each molecule, is the principal component used to form silicone rubber by means of crosslinking the present composition. Component (A) is a substantially linear diorganopolysiloxane or a linear diorganopolysiloxane with some branching represented by average unit formula
RnSiO(4-n)/2,
where R is a monovalent hydrocarbon group exemplified by methyl, ethyl, propyl, butyl, pentyl, hexyl, or other alkyl groups; vinyl, allyl, propenyl, hexenyl, or other alkenyl groups; phenyl, tolyl, or other aryl groups; or 3,3,3-trifluoropropyl, 3,3,3-trichloropropyl, or other halogen-substituted monovalent hydrocarbon groups; the subscript n is a value of 1.9xcx9c2.1. The viscosity of this type of diorganopolysiloxane is typically in the range of from 100 mPaxc2x7s to 1,000,000 mPaxc2x7s.
Component (A) is exemplified by dimethylvinylsiloxy-endblocked dimethylpolysiloxane, dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-methylphenylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane-methylphenylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-diphenylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane-diphenylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-methyl(3,3,3-trifluoropropyl)siloxane copolymers, and dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane-methyl(3,3,3-trifluoropropyl)siloxane copolymers.
Any inorganic filler known in the past as reinforcing fillers or semi-reinforcing fillers can be used as the inorganic filler of component (B), which is used for regulating the viscosity of the present composition and for imparting mechanical strength to the silicone rubber. The reinforcing fillers are exemplified by dry process silicas, wet process silicas, and hydrophobic silicas obtained by surface treating these silicas with, for example, organochlorosilane, organoalkoxysilane, organopolysiloxane, or organosilazane. Silica micropowders with an average particle size of 50 xcexcm or less and a specific surface area by the BET method of at least 100 m2/g are preferred. The semi-reinforcing fillers are exemplified by diatomaceous earth, quartz powder, mica, aluminum oxide, titanium oxide, and materials obtained by treating them with organosilanes, organopolysiloxanes, fatty acids, and the like. Component (B) is compounded into the composition at 5xcx9c300 parts by weight per 100 parts by weight of component (A).
The organohydrogenpolysiloxane of component (C), which has at least two silicon-bonded hydrogen atoms in each molecule, acts as a cross-linking agent for the present composition. The organohydrogenpolysiloxane is exemplified by methylhydrogenpolysiloxane having both terminal ends blocked by trimethylsiloxy groups, dimethylsiloxane-methylhydrogensiloxane copolymers having both terminal ends blocked by trimethylsiloxy groups, dimethylsiloxane-methylhydrogensiloxane copolymers having both terminal ends blocked by dimethylhydrogensiloxy groups, and tetramethyltetrahydrogencyclotetrasiloxane. Component (C) is admixed in a quantity that affords a value of 0.3 to 5, and, preferably, in a quantity that affords a value of 0.3 to 3, for the molar ratio of silicon-bonded hydrogen atoms in component (C) to alkenyl groups in component (A). When the molar ratio is smaller than 0.3, the cross-linking density becomes too low, and the curing does not produce an elastic material, and when it exceeds five, due to a dehydrogenation reaction, bubbles may appear and the heat resistance of the cured rubber may decrease.
The platinum catalyst of component (D) promotes the addition reaction of the above-described component (A) and component (C). The platinum catalyst is exemplified by platinum micropowder, platinum black, chloroplatinic acid, alcohol-modified chloroplatinic acid, chloroplatinic acid/olefin complexes, and chloroplatinic acid/alkenylsiloxane complexes. Component (D) is admixed in a quantity of 0.1xcx9c500 parts by weight per 1,000,000 parts by weight of component (A).
Component (E) is a component imparting excellent high-temperature heat resistance to the present composition. Component (E) is selected from the group consisting of particulate hydroxides or oxides of alkali metals and particulate hydroxides or oxides of alkaline earth metals, for example, by oxides of lithium, sodium, potassium, calcium, strontium, or barium, and by their hydroxides, with calcium hydroxide and calcium oxide as most suitable from the standpoint of their milder basicity. The compounds of component (E) can be used singly or as mixtures of two or more. In addition, micropowders are helpful for efficiently dispersing them in the composition of the present invention. Component (E) is compounded into the present composition at 0.0xcx9c110 parts by weight, and preferably at 0.05xcx9c5 parts by weight, per 100 parts by weight of component (A). If the amount is less than 0.01 parts by weight, it does not impart sufficient heat resistance to the present composition and if it exceeds 10 parts by weight heat resistance is effected adversely.
The present composition comprises the above-described components (A)xcx9c(E). In addition to these components, however, various additives, for example, 1-ethynyl-1-cyclohexanol, 3-methyl-1-pentene-3-ol, benzotriazole, and other cure inhibitors; carbon black, red iron oxide, rare earth oxides, rare earth hydroxides, cerium silanolate, cerium fatty acid salts, and other heat resistance-improving agents; various flame retardants, internal mold release agents, pigments, and the like, can be added thereto so long as the purpose of the present invention is not impaired.
Various mixing devices used in the preparation of silicone rubber compositions, for example, a kneader mixer, a pressure kneader mixer, a ROSS mixer, or a continuous kneader extruder can be used as equipment for preparing the present composition.
Next, explanations are provided regarding the fluorocarbon resin-covered fixing roll of the present invention. The fluorocarbon resin-covered fixing roll of the present invention is obtained by providing a fluorocarbon resin layer on the peripheral surface of a silicone rubber layer formed out of a cured product of the above-mentioned silicone rubber composition. Iron, aluminum, and stainless steel are examples of the raw material for the fabrication of the roll shaft of the fixing roll of the present invention.
In addition, commercially available tubes of fluorocarbon resin or fluorocarbon resin coating agents can be used as the fluorocarbon resin. More specifically, tubes of fluorocarbon resins, such as polytetrafluoroethylene resins (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resins (PFA), fluorinated ethylene propylene copolymer resins (FEP), ethylene-tetrafluoroethylene copolymer resins (ETFE), polychlorotrifluoroethylene copolymer resins (PCTFE), polyvinylidene fluoride resins (PVDF), polyvinyl fluoride resins (PVF), ethylene chlorotrifluoroethylene copolymer resins (ECTFE), and tetrafluoroethylene-hexafluoropropylene copolymer resins (FEP); and fluorocarbon resin coating agents, such as polytetrafluoroethylene resin (PTFE) latex and DAI-EL latex (fluorocarbon resin latex produced by Daikin Industries, Ltd.) are suggested. The thickness of the fluorocarbon resin layer is typically not more than 0.1 mm, preferably, in the range of 0.1xcx9c50 xcexcm, and the thickness of the silicone rubber layer is typically 0.1xcx9c50 mm, preferably, 0.1xcx9c30 mm.
Hereinbelow, the present invention is explained in a more specific manner by referring to working examples and comparative examples. In the working examples, the word xe2x80x9cpartxe2x80x9d means xe2x80x9cparts by weight,xe2x80x9d and the word xe2x80x9cviscosityxe2x80x9d refers to values measured at 25xc2x0 C. In addition, the hardness and compression set of the silicone rubber were measured in the following manner.
Hardness
Silicone rubber sheets with a thickness of 6 mm were fabricated by heating the silicone rubber composition in a hot press at 120xc2x0 C. for 10 minutes followed by heating at 200xc2x0 C. for 4 hours. The hardness of the silicone rubber sheets was measured using a JIS Type A durometer as specified in JIS K 6249.
Compression Set
Test specimens of silicone rubber for compression set measurement with a thickness of 12.7 mm were fabricated by heating the silicone rubber composition in a hot press at 120xc2x0 C. for 15 minutes followed by heating at 200xc2x0 C. for 4 hours. The compression set of the silicone rubber test specimens was measured in accordance with the compression set test method stipulated in JIS K 6249. In addition, during the compression test, the compression ratio was 25%, the heat treatment temperature was 180xc2x0 C., and the heat treatment time was 22 hours.
Heat Resistance
Silicone rubber sheets with a thickness of 6 mm were fabricated by heating the silicone rubber composition in a hot press at 120xc2x0 C. for 10 minutes followed by heating at 200xc2x0 C. for 4 hours. Fluorocarbon resin (fluororubber coating from Daikin Industries, Ltd.; trade name: DAI-EL Latex GLS-213) was uniformly spray-coated onto the surface of the silicone rubber sheets and baked at 350xc2x0 C. for 30 minutes. Next, after cooling to room temperature, the fluorocarbon resin was peeled off. After that, the hardness of the thus obtained baked silicone rubber sheets was measured using a JIS Type A durometer as stipulated in JIS K 6249.